1. Field of the Invention
The present invention relates to a system and method for measuring a chromatic dispersion in an optical fiber, and more particularly, a technology which enables an implementation of a system for an accurate measurement of a chromatic dispersion in both a long-distance optical fiber and a short-distance optical fiber such as an optical fiber device through a proposal of a measurement method for a chromatic dispersion in an optical fiber which is insensitive to an external environmental condition.
2. Description of the Related Art
The chromatic dispersion representing a wavelength dependency of a wave group velocity due to the property of matter and the structure of an optical waveguide is a characteristic of an optical fiber causing a temporal spreading of an optical pulse which allows transmission of data, and a controlled chromatic dispersion is one of main technologies in implementation of a very high-speed and high-capacity optical communication system.
Accordingly, there have been researched various methods for controlling and correctly measuring the chromatic dispersion of an optical fiber having a specific dispersion characteristic such as an optical fiber diffraction grating, a dispersion shifted fiber (hereinafter, referred to as xe2x80x9cDSFxe2x80x9d) or a dispersion flattened fiber (hereinafter, referred to as xe2x80x9cDFFxe2x80x9d).
Particularly, it is important to measure a chromatic dispersion in a long-distance single mode optical fiber which mainly acts as a transmission line for the control of the chromatic dispersion. Recently, there is a need for an accurate measurement of a chromatic dispersion characteristic for respective very short length of optical fiber device along with a development of an optical fiber device such as an optical fiber diffraction grating.
FIG. 1 is a schematic block diagram illustrating the construction of a system for measuring a chromatic dispersion in an optical fiber using a conventional optical fiber Raman laser.
Referring to FIG. 1, there is shown the chromatic dispersion measuring system which includes an optical fiber Raman laser 10, a monochrometer 12, a test optical fiber 14, a photodetector 16, an oscilloscope 18, and an Nd:YAG pump laser 20.
The Nd:YAG pump laser 20 further includes an M-L RF driver, a Q-S RF driver, and a digital delay generator.
This measurement technique is using the time-of-flight method.
First, an optical pulse is generated and the generated optical pulse passes through a test optical fiber 14. The time delay difference between pulses having different wavelengths due to the chromatic dispersion in the test optical fiber is measured by a sampling oscilloscope 18 and a high speed photodetector 16.
A silica optical fiber Raman laser 10 pumped with a mode locked and Q switched Nd:YAG pump laser (xcex=1.06 xcexcm) 20 is used, it is possible to measure a relatively large wavelength band of 1.1xcx9c1.2 xcexcm, but the necessity for the monochrometer 12 makes miniaturization of the chromatic dispersion measuring system difficult.
FIG. 2 is a schematic block diagram illustrating the construction of a system for measuring a chromatic dispersion in an optical fiber using a conventional semiconductor laser diode array.
A construction and work of the chromatic dispersion system shown in FIG. 2 will be described hereinafter briefly.
Referring to FIG. 2, there is shown the chromatic dispersion measuring system which includes an electric pulse source, an InGaAsP semiconductor laser array 30, a wavelength division multiplexer 32, a test optical fiber 34, a high speed photodetector 36 and an oscilloscope 38.
The InGaAsP semiconductor laser array 30 is composed of six InGaAsP semiconductor lasers, which is driven with an electric pulse of 100 ps.
Like this, the use of the InGaAsP semiconductor laser array 30 enables miniaturization of the chromatic dispersion measuring system, but the necessity for the wavelength division multiplexer 32 results in an increase in a cost required for fabricating the chromatic dispersion measuring system.
Also, all the InGaAsP semiconductor lasers must be replaced according to a variation in a wavelength of a interest domain, which causes a difficulty in selecting a wavelength flexibly.
FIG. 3 is a schematic block diagram illustrating the construction of a system for measuring a chromatic dispersion in an optical fiber using a conventional phase shift measurement method.
A construction and work of the chromatic dispersion system shown in FIG. 3 will be described hereinafter briefly.
Referring to FIG. 3, there is shown the chromatic dispersion measuring system which includes a light source 40, a monochrometer 42, a photodetector 44, an amplifier, a vector electrometer 46, a curve fitting section 48, and a signal generator 50.
The phase shift measurement method according to FIG. 3 is a method in which an optical signal modulated with a sinusoidal wave instead of an optical pulse passes through a test optical fiber 34 in which a chromatic dispersion is measured, and then a phase shift for a wavelength caused due to the chromatic dispersion in the optical fiber is measured.
That is, when an optical signal modulated with a frequency f passes through the test optical fiber 34 in which a chromatic dispersion is measured and a phase shift before and after a passage of the optical signal through the test optical fiber 34 is measured according to a wavelength, the phase shift can be written as follows according to a group delay xcfx84 g(xcex) per mode.
"PHgr"(xcex)=2xcfx80fxcfx84g(xcex)
Thus, such a phase shift according to a wavelength is measured and the measured phase shift undergoes a curve fitting process in the curve fitting section 48 so that the chromatic dispersion is estimated.
Accordingly, such a method has a disadvantage in that a phase shift according to a wavelength is measured, and then undergoes the curve fitting process again. Further, instead of a semiconductor laser an LED may be used as a light source 40, which makes it possible to fabricate the chromatic dispersion measuring system at a low cost, but the necessity for the monochrometer 42 makes miniaturization of the chromatic dispersion measuring system difficult.
In addition, in case of the chromatic dispersion measuring system, it is difficult to separate the light source 40 modulated by the signal generator 50 and the vector electrometer 46 for detecting the optical signal which has passes through the test optical fiber 34 from each other, which causes a problem in a remote control of the chromatic dispersion measuring system.
FIG. 4 is a schematic block diagram illustrating the construction of a system for measuring a chromatic dispersion in an optical fiber using a conventional interferometer.
Referring to FIG. 4, there is shown the chromatic dispersion measuring system which includes a white light source 60, a monochrometer 62, a reference optical fiber 64, a test optical fiber 66, and photodetector 68.
In an interferometric measurement method using the interferometer shown in FIG. 4, a Mach-Zehnder interferometer is used as a basic interferometer, and an input light is divided into two optical signals of an identical optical path. One of the two optical beams passes through the reference optical fiber 64 as a reference, and the other passes through the test optical fiber 66.
The two beams (optical pulses) are synthesized. The chromatic dispersion characteristic in the optical fiber to be measured from an interference fringe of the synthesized beam.
In such a method, a very short optical fiber within 1 m is advantageous, but in many cases the white light source 60 is used and the necessity for the monochrometer 62 makes miniaturization of the chromatic dispersion measuring system difficult.
Moreover, there is also needed a data processing process for again calculating a chromatic dispersion from a group delay caused by the chromatic dispersion in the test optical fiber obtained from the interference fringe, and a beam alignment and a chromatic dispersion measurement for a long-distance optical fiber is difficult.
Further, since the optical path of the two beams must be identical, an environment condition for the two beams must also be identical. For this reason, it is difficult to use such a chromatic dispersion measuring system for the long-length of optical fibers in a practical field.
As described above, according to such conventional prior arts (FIGS. 1 to 4) for the chromatic dispersion measurement, an optical pulse is generated and the generated optical pulse passes through an optical fiber in which a chromatic dispersion is measured. After that, a chromatic dispersion characteristic in the optical fiber to be measured can be found out indirectly using both a difference in a time domain or a frequency domain according to each wavelength and an interferometer.
Further, such a chromatic dispersion measurement requires a low-priced and miniaturized chromatic dispersion measurement system to enable the application of the system in a practical field since the chromatic dispersion measurement system is applicable to an optical fiber installed in a practical optical communication system. There is also needed a development of a chromatic dispersion measurement system for an application in a practical system insensitive to an external environment.
Therefore, the present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a system and method for measuring a chromatic dispersion in an optical fiber which enables an implementation of a system for an accurate measurement of a chromatic dispersion in both a long-distance optical fiber acting as a transmission line and a short-distance optical fiber such as an optical fiber element.
According to one aspect of the present invention, there is provided a system for measuring a chromatic dispersion in an optical fiber, comprising:
a multimode laser diode adapted to generate an optical pulse through a gain switching;
a highly dispersive optical fiber adapted to allow the gain switched optical output pulse to pass therethrough, and then adapted to separate each mode of the multimode laser diode to generate the multiwavelength optical pulse train as a reference signal;
a test optical fiber adapted to allow the multiwavelength optical pulse train by the highly dispersive optical fiber to pass therethrough to vary a spacing of the optical pulse due to a chromatic dispersion characteristic of the test optical fiber;
a high speed photodetector adapted to detect the variation of the repetition rate of the multiwavelength optical pulse train due to the chromatic dispersion characteristic of the test optical fiber; and
an RF spectrum analyzer adapted to measure the repetition rate of the multiwavelength optical pulse train in frequency domain);
According to another aspect of the present invention, there is also provided a method for measuring a chromatic dispersion in an optical fiber, comprising the steps of:
(a) generating a pulse train through a gain switching of a multimode laser diode;
(b) generating a reference signal through the use of a mode separation by a highly dispersive optical fiber to generate a multi-wavelength pulse train;
(c) allowing the multi-wavelength pulse train generated by the highly dispersive optical fiber to pass through a test optical fiber; and
(d) measuring a time delay difference between pulses having different wavelengths due to the chromatic dispersion in the optical fiber in a frequency domain.
According to the present invention, there is a proposed method for measuring a chromatic dispersion in an optical fiber in which the use of the generation of an optical pulse through a gain switching of a multimode Fabry Perot-laser diode (hereinafter, referred to as xe2x80x9cFP-LDxe2x80x9d) and the mode separation by a highly dispersive optical fiber generates an multiwavelength optical pulse train as a reference signal.